MURAWSKI ET AI..: GROWTH OF OCEAN QUAHOG. ARCT1CA ISLANDICA 



FIELD STUDIES 



Intermittent surveys of offshore clam re- 

 sources of the Middle Atlantic Bight have been 

 conducted since 1965 by the National Marine 

 Fisheries Service, and its predecessor the Bu- 

 reau of Commercial Fisheries (Merrill and 

 Ropes 1969; Murawski and Serchuk footnote 2; 

 Serchuk et al. 10 ). Cruises were designed to yield 

 information on temporal and areal aspects of dis- 

 tribution, size composition, and relative abun- 

 dance of both surf clam, Spisula solidissima, and 

 ocean quahog. Stations were sampled in a grid 

 array prior to 1978; surveys from 1978 to 1980 

 employed a stratified-random scheme. Commer- 

 cial-type hydraulic clam dredges were modified 

 to retain small individuals and used as survey 

 gear; dredge specifications and vessels varied 

 somewhat among cruises (Serchuk et al. footnote 

 10; Table 1). 



We selected an area for intensive field study of 

 ocean quahog growth, based on an evaluation of 

 pre-1978 survey data and knowledge of commer- 

 cial fleet activities. Specific criteria were: 1) suf- 

 ficient clam densities for rapid capture of indi- 

 viduals used in the marking experiment, 2) 

 abundant numbers of clams over a wide size 

 range, 3) clam densities similar to sites fre- 

 quented by fishing vessels, and 4) lack of pre- 

 vious exploitation and low probability of near- 

 future use. These specifications were met at a 

 site 48 km south-southeast of Shinnecock Inlet, 

 Long Island, at lat. 40°25.1'N, long. 72°23.7'W. 



'"Serchuk, F. M.. S. A. Murawski, E. M. Henderson, and 

 B.E.Brown. 1979. The population dynamics basis for man- 

 agement of offshore surf clam populations in the Middle Atlan- 

 tic. Proceedings of the Northeast Clam Industries - Manage- 

 ment for the Future, Coop. Ext. Serv. Univ. Mass. -MIT Sea 

 Grant, p. 83-101. 



Water depth was 53 m, and substrata consisted of 

 coarse sand and shell, primarily ocean quahog 

 and sea scallop, Placopecten magellanicus. Live 

 invertebrates present in survey samples in- 

 cluded Lunatia heros, Echinarachnius parma, 

 Venericardia borealis, Aphrodite aculeata, and 

 Astarte spp., in addition to ocean quahog and sea 

 scallop. 



Water depth at the study site precluded ex- 

 tended periods of bottom time using normal 

 scuba methods, thus we elected to sample ocean 

 quahogs with commercial and research dredging 

 vessels. The probability of recapturing marked 

 ocean quahogs at the site was considered to be 

 relatively low because of water depth, width of 

 sampling gear, difficulties in positioning the ves- 

 sel at a precise location, and the accuracy of the 

 loran-C navigation system. Hence it was decided 

 to mark and redistribute large numbers. 



Incremental increases in clam shell growth 

 corresponding to known time durations can be 

 measured if a point of reference is initially estab- 

 lished at the margin of the growing shell. Growth 

 is determined directly from recaptured speci- 

 mens and shell length at marking can either be 

 measured or back-calculated. Thus we needed 

 only to indelibly etch the shell edge of live qua- 

 hogs and return them to the sea bed, obviating 

 the laborious and time-consuming process of 

 measuring and number-coding individuals prior 

 to release. 



Notching techniques have been used success- 

 fully to study growth rate and to validate the 

 periodicity of band formation in a number of bi- 

 valve species including soft shell clam, Mya 

 arenaria (Mead and Barnes 1904); hard shell 

 clam, Mercenaria mercenaria (Belding 1912); 

 American oyster, Crassostrea virginica (Loosa- 

 noff and Nomejko 1949); sea scallop (Stevenson 



Table 1. — Characteristics of survey gear and length-frequency statistics of ocean quahogs collected near lat. 

 40°25' N, long. 72°24' W, in the Middle Atlantic Bight, 1970-80. 



'Dimension in the portion of the dredge where catch is accumulated 

 2 Samples measured to the nearest 5 cm. 

 initiation of marking study 

 4 Recapture of marked individuals. 



23 



